When designing and constructing wind turbines it is a significant parameter to the output of the plant, that the energy in the wind is utilized maximally. To this end wind turbines have been provided with various devices for, on the one hand, directing the wind to the rotor of the turbine and, on the other hand, increasing the speed of the wind in front of and behind the rotor by using the so-called venturi effect to advantage. The venturi effect describes the effect that occurs in a flow through a pipe with a narrowed portion, where the flow is accelerated as the cross-section area decreases. Thus, the venturi effect comprises a speed-up effect in front of the narrowed portion and a diffuser effect after where a subatmospheric pressure is generated.
For instance from JP 2004 052721, a wind turbine is known which is equipped with a diffuser or a funnel around the rotor. The shape of the diffuser causes the speed of the wind to increase and hence the effect that can be drawn out of the wind by the diffuser. The principle works well and the output of the turbine can be increased considerably. However, the dimensions of modern wind turbines are now so large with rotor diameters of more than 100 m that a correspondingly large diffuser matching them cannot be realised due to, on the one hand, the increased mass to be carried by the wind turbine tower and, on the other, due to the loads on the funnel becoming, in case of high wind speeds such as eg stormy weather, so powerful that the construction cannot last irrespective of whether the turbine is otherwise turned out of the wind. The latter aspect also applies to smaller wind turbines with smaller rotor diameters.
Moreover, it is known (eg from US 2004/0113431 and CA 2 439 420) to arrange a wind turbine with vertical axis of rotation within a tower structure with vertical shields or shudders that are arranged all the way around the rotor. Here the shields ensure that the turbine is able to operate at all wind directions and, moreover, that they are able to shut off completely or reduce the wind considerably in hard winds and hence serve to protect the turbine. In order to function, the shields or towers have a height that reaches past the rotor and it follows that, likewise, they can be realised only in case of relatively small wind turbines, since, on the one hand, they will make a large wind turbine take up much space in the landscape and, on the other, they will not be able to cope with the loads occurring as a result of high wind speeds.
Moreover, over time it has been attempted to maximize the output of wind turbines by placing them appropriately in the landscape or even by adapting the surrounding landscape to the turbine. Thus, it is known to arrange a wind turbine on a hill whose shape is cultivated and adapted especially to be able to increase the wind speed towards the turbine maximally. However, such cultivation is often far from possible or desirable from the point of view of the land owner.
WO 02/068817 describes placing an artificial mound adjacent a wind turbine, this mound is a static element and cannot be moved during higher wind speeds.
WO 2004/011799 describes positioning of static passive profiles in front of the wind turbine.
In both documents a compromise has to be made to avoid extreme loads on the wind turbine that can damage the wind turbine during extreme winds.